Doping refers to the insertion of atoms into a substrate to achieve a change in the substrate material properties. The properties can include, e.g., electrical mobility, index of refraction, surface reactivity, mechanical properties or more specifically electrical band structure. The most common application of doping is the implant of donor and acceptor species into electrical devices such as gate sources and drains. The process of doping can include an atom infusion step followed by an electrical activation step including the application of heat. Plasma doping is an alternative to low energy ion implantation for the doping of fin structures or other non-planar electrical structures on semiconductor substrates being processed. Electron cyclotron resonance (ECR) plasma and inductively coupled plasma (ICP) plasmas have been used for plasma doping. More recently, microwave plasma sources, particularly surface wave based plasma sources have been used for plasma doping. Typically, plasma doping sources employ center and edge gas injection. This central or peripheral gas injection is employed because using gas injection from showerheads opposite the wafer/substrate is difficult to implement in ECR, ICP, or microwave driven sources.
In doping a substrate, problems can arise in providing conformal or uniform doping across the substrate such as where the surface of the substrate is non-uniformly saturated with dopant across the substrate surface. In certain areas excessive amounts of the dopant can be infused which can cause the dopant species to form clusters. Here, clusters refers to bonded multi-atom (>2) groups of dopant atoms formed on the substrate. Clusters may also be comprised of dopant atoms and other atoms such as oxygen. If clusters of the dopant were formed, they could easily be removed or sublimated after deposition when the substrate is annealed as they become volatile at low temperatures (e.g., about 460 C for Arsenic doping). The consequence is loss of dopant in localized regions and the surface resistivity of the substrate will be non-uniform across the substrate. In addition, if certain areas are insufficiently doped, the surface resistivity of the substrate will be non-uniform across the substrate. Further, certain doping techniques can result in excessively high ion energies, which can undesirably damage features formed on the substrate that are being doped. For example high energy ions can damage the corners of a Fin-FET (fin-like field effect transistors), or excessively penetrate into the substrate creating damaged material (silicon fin or gate structures). Although cleaning steps can remove some damage, excessive damage can exceed amounts that will be removed in cleaning steps, e.g., as used as part of masking n and p (donor and acceptor) regions of die.
Doping of non-planar surfaces is also challenging in part because of poor view factors or geometries. Conformal plasma doping typically uses a dopant flux with an isotropic angular distribution incident on the plane of the wafer or dopant flux with an anisotropic angular distribution typically at 30-45 degrees to the surface normal. For example, with conventional 30-45 degree implantation, equal fluxes of dopant are incident on vertical and horizontal surfaces. This is adequate for structures that are perfectly vertical and horizontal and with vertical surfaces that are not shadowed by neighboring vertical surfaces. Surfaces abutting valleys of aspect ratios greater than one or that are adjacent to relatively tall structures, however, receive relatively few dopants. Examples of these structures include Fin-FET devices, vertical NAND Flash memory structures, and CMOS image sensors. In addition, many structures are not planar and do not possess topographies normal or parallel to the substrate. Examples of these structures include image sensors, photonic devices with raised cylindrical structures, silicon nanowires or gate structures with recesses.